The invention relates to a method of manufacturing optical fibers wherein a preform is heated in a furnace, one end of the preform is melted, a fiber is being drawn from the melting end of the preform, and the fiber is withdrawn from the furnace. In this method, the portions of the preform and of the fiber located in the furnace are flushed with a gas, after which the drawn fiber is cooled and provided with a coating which is subsequently dried. Finally, the fiber obtained is wound onto a reel.
Such a method is known from the article "Preform Fabrication and Fiber Drawing by Western Electric Product Engineering Control Center", published in The Bell System Technical Journal, Vol. 57, No. 6, July-August 1978, pages 1735 to 1744.
For heating the preform this known method employs a graphite resistance element. Owing to its thermal and mechanical properties, i.e. a high thermal shock resistance and a suitable strength at high temperatures, graphite is extremely suitable for this purpose. Moreover, graphite is comparatively cheap, available in pure form and easy to work. However, graphite has the drawback that at the operating temperatures of approximately 2000.degree. C. it is subject to substantial oxidation.
It is known that the drawing conditions may adversely affect the strength of the fiber drawn. Contamination of the fiber by dust particles, owing to the deposition of reaction products from the furnace, such as for example silicon carbide, silica particles etc. will result in damaging and deterioration of the fiber. Contamination of the fiber by dust particles can be avoided by a dust-free environment. The formation of reaction products can be limited by maintaining an inert-gas atmosphere at overpressure in the furnace, so that the admission of air is prevented. Settlement of reaction products on the fiber can be minimized by flushing the fiber with an inert gas stream.